Realization of Organocerium-Based Fullerene Molecular Materials Showing Mott Insulator-Type Behavior

Electron-rich organocerium complexes (C₅Me₄H)₃Ce and [(C₅Me₅)₂Ce(ortho-oxa)], with redox potentials E_1/2 = -0.82 V and E_1/2 = -0.86 V versus Fc/Fc⁺, respectively, were reacted with fullerene (C₆₀) in different stoichiometries to obtain molecular materials. Structurally characterized cocrystals: [(C₅Me₄H)₃Ce]₂·C₆₀ (1) and [(C₅Me₅)₂Ce(ortho-oxa)]₃·C₆₀ (2) of C₆₀ with cerium-based, molecular rare earth precursors are reported for the first time. The extent of charge transfer in 1 and 2 was evaluated using a series of physical measurements: FT-IR, Raman, solid-state UV-vis-NIR spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and magnetic susceptibility measurements. The physical measurements indicate that 1 and 2 comprise the cerium(III) oxidation state, with formally neutral C₆₀ as a cocrystal in both cases. Pressure-dependent periodic density functional theory calculations were performed to study the electronic structure of 1. Inclusion of a Hubbard-U parameter removes Ce f states from the Fermi level, opens up a band gap, and stabilizes FM/AFM magnetic solutions that are isoenergetic because of the large distances between the Ce(III) cations. The electronic structure of this strongly correlated Mott insulator-type system is reminiscent of the well-studied Ce₂O₃.